Well-known solar collectors use parabolic or almost-parabolic mirrors to focus solar radiation energy (or power, which is the time rate of energy) onto one or more collecting tubes which contain an energy-transferring working or heat-transfer fluid. The collecting tubes are often of a metal, such as steel. The tube is assembled in the interior of a glass tube. The glass tube is evacuated to thereby prevent convection heat transfer from the steel collecting tube to the outside world through the glass tube.